In the field of digital video technology, a problem often arises with captured images similar to that encountered in the photographic arts, namely the contrast of the image is defective and in need of correction.
Several methods have been employed in the past to correct such defects. For example, it is known in the satellite image processing art to effectively flatten the gray scale histogram of the image without attempting to sort out and treat differently the artifacts attributable to the image, which are desirable, from those of the image capture process which are undesirable anomalies.
Such technology provides a high entropy image and is quite helpful with respect to images encountered in space exploration. However, when applied to natural images more typically encountered in multimedia work, the result can often include very disturbing and unnatural appearing artifacts associated with the capture process as, for example, in a grainy appearance of the sky in an outdoor image.
Technology has further been developed to permit manual adjustment of contrast levels as, for example. in equipment employed in photographic film processing to produce photographic images. A correction curve employed by the equipment may be adjusted by hand, for example, in a commercial slide service establishment, wherein an operator may raise highlights, lower shadows, raise medium grays, lower middle grays, or the like, so as to enhance or reduce undesirable characteristics of the photograph such as saturation.
In a related area, it has been known to provide various forms of video signal processing equipment which permits the operator to manually adjust the contrast or gamma of the captured video signal. While such manual techniques provide the opportunity for great flexibility and artistic input these same characteristics also give rise to numerous problems. Perhaps the most obvious of these is the need for skilled operators and their subjective judgement, as well as the repeatability and consistency of the artistic judgements which are being made.
It was accordingly highly desirable in some applications, such as in commercial multimedia systems. to make provision for some form of automatic gamma or contrast adjustments in the video image whereby these artistic functions might be performed heuristically without user intervention. Certainly various automatic contrast schemes have been devised in the image processing and photographic arts in seeking to remove the human variable, as, for example, in the automatic range setting of black-white levels or automatic brightness controls in some forms of video equipment. In some cases, for example in commercial photography, an average of gray scales in the image is sometimes taken and the brightness level set as a function of this average. In more sophisticated equipment, automatic processing circuitry may examine the image for the brightest and darkest levels and set the contrast as a function of these levels.
A serious problem with these automated approaches, however, is that again there is no attention given to seeking to discriminate between image artifacts and undesirable capture artifacts. A technique was thus highly desired which could automatically perform basic contrast correction on captured images without user intervention while at the same time removing from the image only the artifacts introduced by the capture process.